JEE Main 2021 · 27 July · morning · Single correct · +4 / −1

The oxidation states of 'P' in H₄P₂O₇, H₄P₂O₅ and H₄P₂O₆, respectively, are :

  1. (a) 7, 5 and 6
  2. (b) 5, 4 and 3
  3. (c) 5, 3 and 4
  4. (d) 6, 4 and 5
Answer and solution

Answer (c) 5, 3 and 4

Solution

Chemistry ma’am · 2 steps

All three acids have 4 H and 2 P, so only the oxygen count changes the answer.

  1. Set up the sum. Take H as +1, O as -2 and P as x. The molecule is neutral, so all oxidation numbers add to zero. Remember there are two P atoms, so P contributes 2x.

    4(+1) + 2x + n(-2) = 0 ⇒ x = (2n - 4)/2 = n - 2

    AcidO atoms, nEquationP state
    H₄P₂O₇74 + 2x - 14 = 0+5
    H₄P₂O₅54 + 2x - 10 = 0+3
    H₄P₂O₆64 + 2x - 12 = 0+4
  2. So. In the order asked, P is +5, +3 and +4, which is option (c).

The idea to keep. When a formula has two atoms of the same element, write 2x, not x. Then divide the leftover charge between them.

Answer from the NTA final answer key. Solution by PrepWiser.

Study Redox Reactions

Asked about 90 times in JEE Main since 2020. This question is from 2021.